FORT SASKATCHEWAN, ALTA-A process upset at Shell Canada's Scotford heavy oil upgrader plant near Fort Saskatchewan resulted in a release of gas containing small quantities of hydrogen sulfide (H2S) and sulfur dioxide (SO2). The release occurred around mid-morning on September 12 when a leak developed on a bypass valve in the upgrader's residue hydroconversion (RHC) unit, which upgrades bitumen into synthetic crude oil. The unit was subsequently taken out of service. As part of its emergency response plan, Shell called local residents within a five-kilometre radius of the facility, advising those downwind (east of Scotford) to take stay indoors. About 1,400 employees and contractors from the refinery, upgrader and chemical plant were also evacuated to safe locations within the Scotford complex site. Air monitoring stations and mobile equipment measured no levels above 0.1 parts per million (ppm) of SO2, H2S or hydrocarbon emissions outside of the plant site, although some odours were reported into the afternoon. Steam and vapourized hydrocarbons were visible over the plant site for approximately three hours. By mid-afternoon, the leak had been stopped and a safe shutdown of the RHC unit continued. There were no injuries. Starting in the early afternoon when it was safe to do so, non-essential personnel were permitted to go home. By mid-afternoon, Shell confirmed that an "all clear" had been issued to neighbours. Regulatory agencies were notified and were on-site for the duration of the incident. The Alberta Energy and Utilities Board (EUB) is investigating the cause of this incident, as well as an unrelated upset that occurred at the plant on September 7. As well, Peter St. George, general manager of the complex, conveyed the company's apologies to area residents "for any inconvenience or concern that these events have caused."